This invention relates to a television camera tube device and more particularly to an improvement thereof wherein a laminar flow electron beam is generated under a normal operation and a high beam current can be obtained as necessary.
In a vidicon type television camera tube, a pattern of electric charges corresponding to a level of brightness of the object is formed on a photoconductive layer, the photoconductive layer is scanned with an electron beam emitted from an electron gun to sequentially discharge the patterned electric charges, and a charging current corresponding to the sequential discharging is delivered out of the tube as a signal. The whole amount of the electric charge once generated in accordance with the object is not usually discharged completely during one cycle of beam scanning. Consequently, even when the object disappears from view of the tube, a false signal corresponding to a residual electric charge is generated during the ensuing cycles of beam scanning to produce a signal lag and hence the quality of the picture is degraded when a moving object is picked up.
Particularly, in a television camera tube with a blocking type photoconductive layer, the signal lag is mainly due to a capacitive signal lag having a time constant which is determined by a product of an electrostatic capacitance of the photoconductive layer and a beam resistance of the scanning electron beam. The beam resistance is equivalent to a velocity distribution of electrons which form the electron beam and in order to realize a low lag characteristic, the electron beam is required to have a narrow velocity distribution of electrons.
As well known in the art, the electrons emitted from the cathode electrode have a velocity distribution subject to a Maxwellian distribution but when the electrons are converged to a narrow beam having an increased current density, an energy relaxation phenomenon due to coulomb force interaction between the electrons takes place to broaden the velocity distribution. This phenomenon is called the Boersh effect, and as also well known in the art, the broadening rate of the velocity distribution is approximately proportional to J(z).sup.1/3, where J(z) represents current density on the beam axis.
Accordingly, in a television camera tube aiming at the low lag characteristic, an increase in the beam current density must be suppressed as far as possible. To this end, a diode type electron gun has been proposed (for example, in U.S. Pat. No. 3,894,261) wherein a first grid electrode opposing a cathode electrode is connected to receive a positive voltage relative to the cathode electrode so as to cause electrons to be emitted from the cathode electrode in parallel with the tube axis, thereby generating a laminar flow electron beam which does not form a crossover where current density is high. In this type of diode type electron gun for generation of the laminar flow electron beam, however, the amount of beam current is proportional to the emission current density from the cathode electrode and therefore in order to obtain a high beam current, it is necessary to increase the current density emitted by the cathode electrode to an extreme and accordingly there arise difficulties in expanding the dynamic range of the beam current amount for the sake of operating an automatic beam optimizer (hereinafter referred to as ABO) which controls the amount of beam current in accordance with a level of brightness of the object.